Preliminary analysis of early data returned by NASA's historic Galileo
probe mission into Jupiter's atmosphere has provided a series of startling
discoveries for project scientists.

Information on the extent of water and clouds and on the chemical composition
of the Jovian atmosphere is particularly revealing. Probe instruments found
the entry region of Jupiter to be drier than anticipated, and they did not
detect the three-tiered cloud structure that most researchers had postulated.
The amount of helium measured was about one-half of what was expected.

These initial findings are encouraging scientists to rethink their theories
of Jupiter's formation and the nature of planetary evolution processes, according
to probe project scientist Dr. Richard Young of NASA's Ames Research Center,
Mountain View, CA.

"The quality of the Galileo probe data exceeds all of our most optimistic
predictions," said Dr. Wesley Huntress, NASA Associate Administrator for
Space Science. "It will allow the scientific community to develop valuable
new insights into the formation and evolution of our solar system, and the
origins of life within it."

The probe made the most difficult planetary atmospheric entry ever attempted,
according to probe manager Marcie Smith of NASA Ames. Entering Jupiter's
atmosphere on Dec. 7, 1995, it survived entry speeds of over 106,000 mph,
temperatures twice those on the surface of the Sun and deceleration forces
up to 230 times the strength of gravity on Earth. It relayed data obtained
during its 57-minute descent mission back to the Galileo orbiter more than
130,000 miles overhead for storage and transmission to Earth. The orbiter
is now embarking on a two-year mission to study Jupiter and its moons.

"The probe detected extremely strong winds and very intense turbulence
during its descent through Jupiter's thick atmosphere. This provides evidence
that the energy source driving much of Jupiter's distinctive circulation
phenomena is probably heat escaping from the deep interior of the planet,"
Young said.
"The probe also discovered an intense new radiation
belt approximately 31,000 miles above Jupiter's cloud tops, and a veritable
absence of lightning."

The composition of Jupiter's atmosphere offered some surprises, according
to project scientists. It contains significantly lower-than-expected levels
of helium, neon, and certain heavy elements, such as carbon, oxygen and sulfur.

The issue of the colors of Jupiter's atmosphere has been much-debated,
but no consensus has developed from probe data to date. The probe encountered
no solid objects or surfaces during it's entire 373-mile (600 km) journey.
This was as expected for a gas-giant planet such as Jupiter.

What are the implications of these findings? Most scientists believe
that Jupiter has a bulk composition similar to that of the gas and dust cloud
of the primitive solar nebula from which the planets and our Sun were formed,
with added heavy elements from comets and meteorites. The probe's measurements
may necessitate a re-evaluation of existing views of how Jupiter evolved
from the solar nebula. For example, the lower-than-expected helium and neon
levels on Jupiter relative to the Sun influence scientific understanding
of the process of fractionation, the "raining out" of helium and neon during
planetary evolution.

During the probe's high-speed, atmospheric-entry phase, deceleration
measurements high in the atmosphere showed atmospheric density to be much
greater than expected. Corresponding temperatures were also much higher
than predicted. The high temperatures appear to require an unidentified
heating mechanism for this region of the atmosphere.

Following probe parachute deployment, six science instruments on the
probe collected data throughout 97 miles (156 km) of the descent. During
that time, the probe endured severe winds, periods of intense cold and heat
and strong turbulence. The extreme temperatures and pressures of the Jovian
environment eventually caused the probe communications subsystem to terminate
data transmission operations.

Earth-based telescopic observations suggest that the probe entry site
may well have been one of the least cloudy areas on Jupiter. At this location,
the probe did not detect the three distinct layers of clouds (a topmost layer
of ammonia crystals, a middle layer of ammonium hydrosulfide, and a final,
thick layer of water and ice crystals) that researchers had anticipated.

Some indication of a high-level ammonia ice cloud was detected by the
net flux radiometer. Evidence for a thin cloud which might be the postulated
ammonium hydrosulfide cloud was provided by the nephelometer experiment.
There was no data to suggest the presence of water clouds of any significance.
The vertical temperature gradient obtained by the atmospheric structure
instrument was characteristic of a dry atmosphere, free of condensation.
Only the one, distinctive cloud structure was identified, and that was of
modest proportion.

The latest analyses of data from the Voyager spacecraft that flew by
Jupiter in 1979 had suggested a water abundance for the planet of twice the
solar level (based on the Sun's oxygen content). Observations of the propagation
of atmospheric waves across Jupiter's cloud tops from the Comet Shoemaker-Levy
9 impacts implied that Jupiter might have a water content of ten times the
solar level. Actual probe measurements, while subject to scientific debate,
suggest a level near that of the Sun. Scientists are left to wonder, "where
is the oxygen?," "where is the water?," and to reconsider their interpretation
of the S-L 9 impacts.

Scientists had expected to find severe winds on Jupiter ranging up to
220 mph. However, the probe appears to have detected winds far greater,
perhaps up to 330 mph. The winds remained fairly constant as the probe descended
deep into the Jovian atmosphere. This suggests that Jupiter's winds are
not caused by differential sunlight at the equator versus the poles or by
heat released by water condensation as on Earth, according to project scientists.

"The origin of Jupiter's winds appears to be the internal heat source
which radiates energy up into the atmosphere from the planet's deep interior,"
Young said. "This impacts Jupiter's climate and circulation patterns, and
suggests a jet stream-like mechanism rather than swirling hurricane or
tornado-like storms."

The probe found that lightning occurs on Jupiter only about one-tenth
as often as on Earth. This is puzzling, but consistent with the absence
of water clouds. A virtual absence of lightning reduces the probability
of finding complex organic molecules in Jupiter's atmosphere, particularly
given its hostile, predominantly hydrogen composition.

Scientists caution that results obtained to date, while dramatic and
exciting, are only preliminary and subject to much further analysis and
refinement. Data transmission problems associated with solar conjunction
between the Earth and Jupiter, the need to refine estimates based on probe
and orbiter trajectories, the presence of higher than anticipated instrument
temperatures, and the need for improved calibration all require a cautious
approach to these early findings.